Hydrosilylation chemistry, typically involving a reaction between a silyl hydride and an unsaturated organic group, is the basis for synthetic routes to produce commercial silicone-based products like silicone surfactants, silicone fluids and silanes as well as many addition cured products like sealants, adhesives, and coatings. Typical hydrosilylation reactions use precious metal catalysts to catalyze the addition of a silyl-hydride (Si—H) to an unsaturated group, such as an olefin. In these reactions, the resulting product is a silyl-substituted, saturated compound. In most of these cases, the addition of the silyl group proceeds in an anti-Markovnikov manner, i.e., to the less substituted carbon atom of the unsaturated group. Most precious metal catalyzed hydrosilylations only work well with terminally unsaturated olefins, as internal unsaturations are generally non-reactive or only poorly reactive. There are currently only limited commercially viable methods for the general silylation of olefins where after the addition of the Si—H group there still remains an unsaturation in the original substrate. This reaction, termed a dehydrogenative silylation, has potential uses in the synthesis of new silicone materials, such as silanes, silicone fluids, crosslinked silicone elastomers, and silylated or silicone-crosslinked organic polymers such as polyolefins, unsaturated polyesters, and the like.
Various precious metal complex catalysts are known in the art including a platinum complex containing unsaturated siloxanes as ligands, which is known in the art as Karstedt's catalyst. Other platinum-based hydrosilylation catalysts include Ashby's catalyst, Lamoreaux's catalyst, and Speier's catalyst.
Other metal-based catalysts have been explored including, for example, rhodium complexes, iridium complexes, palladium complexes and even first-row transition metal-based catalysts to promote limited hydrosilylations and dehydrogenative silylations.
U.S. Pat. No. 5,955,555 discloses the synthesis of certain iron or cobalt pyridine di-imine (PDI) complexes bearing anionic ligands. The preferred anions are chloride, bromide, and tetrafluoroborate. U.S. Pat. No. 7,442,819 discloses iron and cobalt complexes of certain tricyclic ligands containing a “pyridine” ring substituted with two imino groups. U.S. Pat. Nos. 6,461,994, 6,657,026 and 7,148,304 disclose several catalyst systems containing certain transitional metal-PDI complexes. U.S. Pat. No. 7,053,020 discloses a catalyst system containing, inter alia, one or more bisarylimino pyridine iron or cobalt catalyst. Chirik et al describe bisarylimino pyridine cobalt complexes with anionic ligands (Inorg. Chem. 2010, 49, 6110 and JACS. 2010, 132, 1676.) However, the catalysts and catalyst systems disclosed in these references are described for use in the context of olefin hydrogenation, polymerizations and/or oligomerisations, not in the context of dehydrogenative silylation reactions. U.S. Pat. No. 8,236,915 discloses hydrosilylation using Mn, Fe, Co, and Ni catalysts containing pyridinediimine ligands. However, these catalysts are sensitive to air and must be handled in a glove box.
There is a continuing need in the silylation industry for non-precious metal-based catalysts that are effective for efficiently and selectively catalyzing hydrosilylation and/or dehydrogenative silylations. There is also a need for metal-based catalysts that are air stable. Many metal-based catalysts, including those based on iron or cobalt, are not stable under atmospheric conditions. This makes such materials generally unsuitable for application on a production or industrial scale.
Further, many industrially important homogeneous metal catalysts suffer from the drawback that following consumption of the first charge of substrates, the catalytically active metal is lost to aggregation and agglomeration and its beneficial catalytic properties are substantially diminished via colloid formation or precipitation. This is a costly loss, especially for noble metals such as Pt. Heterogeneous catalysts are used to alleviate this problem but have limited use for polymers and also have lower activity than homogeneous counterparts. For example, the two primary homogeneous catalysts for hydrosilylation, Speier's and Karstedt's, often lose activity after catalyzing a charge of olefin and silyl- or siloxyhydride reaction. If a single charge of the homogeneous catalyst could be re-used for multiple charges of substrates, then catalyst and process cost advantages would be significant.